1) Field of the Invention
The present invention relates to a technology for controlling a belt driving unit for an image forming apparatus, and a method of switching a control loop for the belt driving unit.
2) Description of the Related Art
Recently, many of image forming apparatuses such as a copier and a printer that can form a full color image are available, due to marketing needs. The image forming apparatus that can form color images includes, for example, a so-called tandem type image forming apparatus in which a plurality of photoconductors are aligned, and a developing apparatus that develops an image with a different color toner is respectively provided corresponding to the respective photoconductors, a single color toner image-is respectively formed on the respective photoconductors, and the single color toner images are sequentially transferred onto a belt-like or drum-like intermediate transfer unit, thereby forming a full-color synthesized color image.
The tandem type image forming apparatus includes the one of a direct transfer type, as shown in FIG. 17, in which toner images on respective photoconductors 91Y, 91M, 91C, and 91K arranged collinearly are sequentially transferred on a sheet P supported and carried on a sheet carrier belt 93 rotating in a direction of arrow A by respective transfer apparatus 92, to form a full color image on the sheet P, and the one of an indirect transfer type, as shown in FIG. 18, in which toner images on the respective photoconductors 91Y, 91M, 91C, and 91 K are sequentially transferred onto an intermediate transfer belt 94 rotating in a direction of arrow B so as to be superposed on each other, and the images on the intermediate transfer belt 94 are collectively transferred onto the sheet P by a secondary transfer apparatus 95.
The tandem type color image forming apparatus using the intermediate transfer belt, for example as shown in FIG. 18, forms a color image by superposing toner images of different colors formed on the respective photoconductors on the intermediate transfer belt. Accordingly, when the positions at which the respective color images are superposed misaligned from each other, out of color registration and change in delicate hue occur in the image, thereby deteriorating the image quality. Therefore, misalignment (out of color alignment) of the respective color toner images becomes an important problem.
The conventional apparatus that drives the transfer belt includes, for example, those disclosed in Japanese Patent Application Laid-open No. H10-232566, Japanese Patent Application Laid-open No. 2002-91264, and Japanese Patent Application Laid-open No. H11-24507.
The image forming apparatus disclosed in Japanese Patent Application Laid-open No. H10-232566 detects slits provided over the whole circumference of a transfer belt rotated by belt driving unit rollers by a sensor unit, being a first detector, encodes the speed of revolution of the belt driving unit rollers rotatably supporting the transfer belt by a second encoder, compares the speed of revolution of the belt driving unit rollers obtained by the second encoder with the belt speed detected by the sensor unit and encoded by a first encoder to calculate the slippage of the transfer belt, and corrects and controls the speed of revolution of the belt driving unit rollers so that a phase difference between the two encoders becomes zero.
The Japanese Patent Application Laid-open No. 2002-91264 discloses an image forming apparatus that reads a portion that does not include a gap between scales provided on an intermediate transfer belt along the circumferential direction by a sensor, and feedback-controls the intermediate transfer belt based on the read information.
The Japanese Patent Application Laid-open No. H11-24507 discloses a color copier in which an intermediate transfer belt (transfer belt) is rotatably laid across in a tensioned condition between five support rollers including one drive roller, and four color toner images of cyan, magenta, yellow, and black are sequentially superposed on and transferred onto the outer circumference of the intermediate transfer belt, to form a full color image.
A scale formed with fine and precise divisions is provided on the inner surface of the intermediate transfer belt in this color copier, and the scale is read by an optical detector (sensor) to accurately detect a shift position of the intermediate transfer belt. The detected shift position is feedback-controlled by a feedback control system so that the intermediate transfer belt comes to an accurate shift position.
However, the apparatus disclosed in the Japanese Patent Application Laid-open No. H10-232566 has a problem in that when the first encoder that detects the slits and encodes a signal output by the sensor unit does not normally function due to a stain or a damage, or due to deterioration over time, the position (speed) of the transfer belt cannot be corrected and controlled, since there is no other control correction unit to substitute.
The apparatus disclosed in the Japanese Patent Application Laid-open No. 2002-91264 has also a problem such that when detection cannot be performed precisely because the scale or the sensor that detects the scale is stained or damaged, or due to deterioration, the position of the transfer belt cannot be corrected and controlled, since there is no other control loop to substitute.
Further, the apparatus disclosed in the Japanese Patent Application Laid-open No. H11-24507 has a problem in that when the scale is worn out or damaged, or toner or the like adheres on the scale to stain the scale, the sensor may detection error the divisions on the scale, thereby making normal belt position control impossible.